It is possible that the absence of a gestational sac on an ultrasound at 6 weeks could indicate a miscarriage, but it is not definitive. There are several factors that could contribute to this, such as an incorrect estimation of pregnancy dates or an ectopic pregnancy. It is recommended to follow up with your healthcare provider for further evaluation and possibly repeat the ultrasound in order to get a clearer picture of the situation. They may also suggest additional tests, such as blood work or another type of imaging, to gather more information. Remember that every pregnancy is unique, and it's important to consult with your healthcare provider for personalized advice and guidance during this time.